/Engine/Source/Runtime/GeometryCore/Public/PlaneTypes.h
Compute d = Dot(N,P)-c where N is the plane normal and c is the plane constant. This is a signed distance. The sign of the return value is positive if the point is on the positive side of the plane, negative if the point is on the negative side, and zero if the point is on the plane.
double DistanceTo ( const [UE::Math::TVector](API\Runtime\Core\Math\TVector)< RealType > & P ) const